Effects of adding
nano-wollastonite, date palm prunings and two types of resins on the physical
and mechanical properties of medium-density fibreboard (MDF) made from wood
fibres

The effects of adding
nano-wollastonite (NW) and date palm prunings on the physical and mechanical
properties of medium-density fibreboard (MDF) were studied here.
Urea-formaldehyde (UF) and isocyanate (IC) resins were used to produce panels
at contents of 10% and 5% respectively, based on the dry weight of the
composite material. NW was used at the 5% and 10% concentrations based on the
dry weight of the resins, and the results were compared with those for panels
containing no NW. NW with a <100 nm particle size were used. NW was mixed
and sprayed onto the material prior to formation of the mat. The results
indicated distinctly lower water absorption and thickness swelling of panels
produced with IC resin. Adding palm residues significantly increased the IB
values in panels with both UF and IC resins. Adding NW had the effect of
decreasing the mechanical properties of panels produced with UF resin, but had
an increasing effect on panels made with IC resin. It was concluded that palm
leaf residues can be considered as a potential raw material for producing MDF
panels using both UF and IC resins. Moreover, NW is recommended as an additive
in composite panels with 10% of palm residues to compensate in part for the
property losses.

Palm orchard in Bushehr
province in Iran (A); a palm grove with some dried leaved to be pruned (B);
the furnish after former to be hot-pressed (C).

Introduction

Trees produce valuable constructional
materials with unique properties; they help humankind keep sustainable
development; and they provide beauty and clean air for both human and wildlife
(Daly-Hassen et al., 2014; Arce and Moya, 2015; Gbètoho et al.,
2017). Moreover, they provide raw materials for many other products and
composites (Altuntas et al., 2017; Fernandes et al., 2017; Hubbe et
al., 2017). However, it has also some drawbacks, like dimensional
instability, vulnerability to biological deteriorating agents, and fire
(Adamopoulos et al., 2012; Chan-Hom et al., 2017; Esmailpour et
al., 2017; He et al., 2016; Hill, 2006; de Medeiros et al.,
2016; Schmidt, 2006; Schmidt et al., 2016), and therefore, many
modification methods were studied to overcome the drawbacks (Bastani et al.,
2016; Hosseinpourpia et al., 2016, 2017; Behr et al., 2017). One
other drawback is that boards with large dimensions are not readily available
at large quantities due to limitation in time and space for cultivation and
harvesting of large trees (Fernandez-Puratich and Oliver-Villanueva, 2014;
Andrade et al., 2016). A suitable substitute for wood boards is wood
composite panels. Therefore, composite panels were also elaborated from
different aspects as a substitution (Valenzuela et al., 2012; Candan and
Akbulut, 2014; Tajvidi et al., 2016; Lu et al., 2017). In this
connection, different resins and modified resins were also studied to improve
the qualities of wood composite panels (Sheikholeslami et al., 2016;
Mantanis et al., 2017).

The constant need of composite
manufacturing factories for raw materials made way to the utilization of a
variety of lignocellulosic materials as well as different natural and synthetic
fibers. In this connection, camel-thorn, different nuts, pruned branches,
straws, kenaf, and even chicken-feathers were reported to be successfully used
in composite production (Parkinson, 1998; Koch, 2006). However, the main
portion of composite panels consists of wood chips or fibers from fast-growing
trees and trees with lower wood quality (Mendes et al., 2013; Behling et
al., 2017).

Iran is not rich in forest land but
220,000 hectares of date palm cultivation out of 770,000 hectares around the
world are located in Iran (Hosseinkhani, 2015). Date palm leaves should be pruned
regularly to keep them healthy and fertile; in fact, each date palm tree
produces 10-20 kg pruned residues annually (Hosseinkhani, 2015), making an
abundant source of lignocellulosic raw material to be used for industrial
purposes. Date palm pruned residues were reported to be successful in
production of MDF at pilot plant scale and using 100% pruned leaves of date
palm (Hosseinkhani, 2015). However, Iran’s MDF manufacturing plants are
designed for production of MDF with wood fibers; therefore, a percentage of
date palm pruned leaves to be mixed with wood fibers would be a practical
method to provide a portion of the raw material in each production batch, and
to use the same machinery for MDF production at the same time. Similar
procedure was reported to have promising results for Camel-thorn (Alhagi
maurorum) fibers in MDF (Taghiyari et al., 2016a).

Nanotechnology and nanomaterials have
been effective to improve properties of many materials and overcome some of
their drawbacks (Majidi, 2016; Matinise et al., 2016; Harsini et al.,
2017; Pethig, 2017; Suganya et al., 2017). In this regard,
nano-wollastonite (NW) was reported to improve thermal conductivity coefficient
of composite mats (Taghiyari et al., 2013ab), physical and mechanical
properties, biological resistance against wood-deteriorating fungi, and fire
resistance of solid wood species and composites (Karimi et al., 2013;
Taghiyari et al., 2014ab). NW was also reported to make bonds with wood
chemical components (Taghiyari et al., 2016b). Moreover, it was reported
that wollastonite has no health hazards for human or wildlife (Huuskonen et
al., 1983ab; Maxim and McConnell, 2005; Taghiyari and Sarvari Samadi,
2016). Therefore, separate sets of panels were produced with NW mixed in the
resin before being sprayed on the composite furnish.

Experimental methods

Composite panel production

A dry process was used to produce the
MDF panels. In order to do this, industrial defibrated fibers were purchased
from Sanaye Choobe Khazar Company, Iran. The fibers were composed of beech (Fagus
orientalis), alder (Alnus glutinosa), maple (Acer hyrcanum),
hornbeam (Carpinus betulus), and three poplar species (Populus nigra,
Populus deltoides, and Populus euroamericana). The resin and NW had
solid contents of 60% and 40%, respectively. The resin/NW mixture was sprayed
on the wood fibers and thoroughly mixed in a rotating drum before hot pressing.
Average moisture content at the time of pressing was 7.5%. The blended fibers
were manually poured into a 45 × 45 cm frame to form the mat which
was then pressed at 170° C and 160 bar for 4 minutes to produce 16 mm
thick panels with a target density of 700 kg/m3. Four replicate
boards were made for each treatment and the panels were conditioned at 25±2° C
and 42±3% relative humidity for three weeks after pressing. The outer 40 mm
around each panel was cut away before the final specimens were cut to eliminate
source of bias as a result of lower consolidated panel material that might not
be representative of each treatment panel.

Palm leave preparation

Date palm pruned leaves from Phoenix
dactylifera L. were cut and dried from Borazjan city (Dashtestan) located
in Bushehr province in Iran (latitude: 29º 16’ 11” N, longitude: 51º 13’ 7” E,
elevation above sea level: 70 m), and they were taken to Shahid Rajaee Teacher
Training University for defibration (figure 1). They were first washed and then
cooked in boiling water for two hours before being defibrated in refiner in two
runs. The defibrated palm fibers were added to wood fibers at 10% w/w basis.
Separate sets of MDF panels were produced with 0% palm content as control
specimens.

Wollastonite and resin application

Nano-Wollastonite (NW) emulsion with
solid content of 40% and particle size of < 100 nm was purchased from
Mehrabadi Machinery (Mehrabadi Machinery Mfg. Co., Iran); the chemical
composition of NW is explained in table I. NW was blended with urea
formaldehyde (UF) and isocyanate (IC) resins at three levels (0, 5, and
10 % w/w) based on the dry weight of resins. These two NW-content levels
were chosen based on previous projects (Taghiyari et al., 2014ab;
Taghiyari and Sarvari Samadi, 2016). The resin contents were 10% and 5% for UF
and IC resins, respectively, based on the dry weight bases of the fiber content
of composite panels. The UF resin was purchased from Sari Resin Manufacturing
Company, Sari, Iran. It contained 10% of UF with a viscosity of 200-400 cP,
47 seconds of gel time, and a density of 1.277 g/cm3.

Table I.

Composition of the Wollastonite
Gel (Taghiyari et al., 2013bc) evaluated in the study.

Physical and mechanical testing

Specimens were cut from the panels for
physical and mechanical tests in accordance with the dimensions and procedures
described in ISIRI Standard 9044 PB Type 2; this standard is compatible to
procedures described in ASTM Standard D-1037 (table II). Two specimens for each
physical and mechanical tests were cut from each panel replicate.

Center point loading tests were carried
out on an Instron 4486 Universal Testing machine over a 380 mm span. The
loading speed was adjusted at a rate of 2 mm per minute; data were registered
to calculate modulus of elasticity (MOE) and modulus of rupture (MOR).

Internal bond strength was determined
by gluing aluminum blocks to each face of the test sample using hot-melt glue;
pulling force was then applied to the aluminum blocks. Internal bond strength
(IB) was calculated as the maximum load required to fail the sample over the
surface area.

Water absorption was measured by
weighing samples before and after 2 or 24 hours of soaking in distilled water.
Thickness swelling was determined on the same samples by measuring thickness at
five pre-determined points on each sample before and after the water immersion.
The five points comprised of four points on each corner, and one in the center.

Statistical analysis

A two-way analysis of variance (ANOVA)
was carried out using SAS software, version 9.2 (2010) at a 95% level of
confidence. Duncan multiple range test was then performed to determine
groupings for each properties. Hierarchical cluster analysis, including
dendrograms and Ward methods with squared Euclidean distance intervals, was
carried out using SPSS/18 (2010). Fitted-line, contour, and surface plots were
made using Minitab software, version 16.2.2 (2010) (Taghiyari et al.,
2017).

The highest and lowest modulus of
rupture (MOR) values were found in the W100-IC (16.3 MPa) and NW10%-IC (9 MPa)
treatments, respectively (figure 2). Addition of palm fibers to panels also
resulted in a decrease in MOR values of panels without NW content and in panels
produced both with UF and IC resins. This was as a result of weaker strength of
palm fibers. Addition of NW resulted in a decrease in MOR values in panels
without date palm pruning residues, produced with both UF and IC resins. This
was because NW particles absorbed part of the resin around themselves
(Taghiyari et al., 2013a), preventing this part of resin to be actively
involved in the process of sticking fibers together. However, addition of NW
had an improving effect on MOR values in panels with 10% palm fiber content. It
is hypothesized that NW could better be integrated with palm fibers; however,
chemical studies should be carried out to finalize the reasons for this
improvement.

Results of the modulus of elasticity
(MOE) tests demonstrated that the highest (1,411 MPa) and lowest (1,090 MPa)
MOE values were found in the same treatments as the highest and lowest MOR
values (figure 3). The addition of NW resulted in a decrease in panels without
palm residues, and in an increase in MOE values of panels containing palm
fibers; the fluctuations were not statistically significant in all cases. The
increasing and decreasing trends were similar to MOR values, though the proportions
were not the same for the addition of NW5% and NW10%.

The highest and lowest internal bond
(IB) values were found in the Palm10%-NW10% with UF resin (0.43 MPa) and
W100%-NW5% with IC resin (0.21 MPa) panels, respectively (figure 4). A general
trend was obvious in that the addition of palm pruning residues resulted in an
increase in IB values. This was as a result of higher slender ratio of palm
leaves fibers (Hosseinkhani, 2015), making the core section of panels more
integrated and more strongly bonded together. Similar improvement in IB values
was also reported in MDF panels containing camel-thorn chips with higher
slender ratio (Taghiyari et al., 2016a). However, no clear trend was
found as a result of the addition of NW or palm residues, indicating that two
mechanisms were simultaneously interacting. The first mechanism was the
formation of bonds between NW with cellulosic components of wood and palm
fibers (Taghiyari et al., 2016b), resulting in an increase in IB values.
The second mechanism was the absorption of resin by NW particles, resulting in
a decrease in IB values.

Results of WA measurement showed
distinct difference between panels produced with IC resin in comparison to
their UF-resin counterparts (figure 5). Addition of NW tended to decrease WA
both in panels with IC and UF resins; the decreasing trend was more obvious in
IC panels. This decrease was attributed to the formation of bonds between NW
components with wood cell wall polymers (Taghiyari et al., 2016b).
Addition of palm to the mat did not have significant effect on WA values. As to
the panels produced with IC resin, addition of NW and palm did not
significantly affect WA values.

Addition of NW and palm residues has
nearly the same effects on thickness swelling (TS) values as they had on WA
values (figure 6). TS values also illustrated distinctly lower values for
panels produced with IC-resin in comparison to panels with UF-resin, though
IC-resin content was only half of UF-resin content in all treatments. In this
connection, IC is classified as a WBP (weather and boil proof) adhesive;
therefore, the significant difference between IC and UF demonstrated
significant effect of resin type on the overall physical properties of
composite panels.

Cluster analysis of the twelve
treatments based on the physical properties showed clear distinct grouping of
treatments with IC-resin versus treatments with UF-resin (figure 7A).
Within-group clustering of either of resins showed very close grouping of the
treatments (figure 7A). This implied the addition of NW and palm residues did
not significantly affect dimensional stability and water absorption of panels.
However, cluster analysis based on all physical and mechanical properties
showed that although there was a clear distinction between the two resins, but
within-group variations also occurred (figure 7B). This indicated that opposite
to physical properties, mechanical properties of composite panels depended on a
variety of factors involved in the production process like NW-content and palm
residues. In other words, addition of NW and palm residues had significant
effect on the overall mechanical properties of the panels; and they had both increasing
and decreasing effects on different treatments, depending on the components of
panels.

Figure 7.

Cluster analysis of the
twelve treatments based on only physical (A) and all physical and mechanical
properties (B) (IC=isocyanate resin; UF=urea-formaldehyde resin; W=wood;
P=palm residues; N=nanowollastonite).

Contour plot of IB values versus WA24
and TS24 clearly showed an inverse relationship with both WA24 and TS24
properties (figure 8A). Surface plot also demonstrated decreasing trends of
both WA24 and TS24 properties as the IB values increased (figure 9). Contour
plots of IB versus MOR and MOE values illustrated that IB had a direct
relationship with MOR values, indicating that as IB increased, so did MOR
values. However, no particular trend could be observed between IB and MOE
values. Fitted-line plot between WA24 and TS24 values showed rather high and
statistically significant Re-square (77%) between the two properties (figure
10). However, neither high and nor significant R-square values were found
between any other properties studied in this research project.

Figure 8.

Contour plots between
internal bond (IB) versus water absorption (WA) and thickness swelling (TS)
values after 24 hours of immersion in water (A), and between IB versus
modulus of rupture (MOR) and modulus of elasticity (MOE) values (B).

Fitted-line plot
between water absorption (WA) and thickness swelling (TS) values after 24
hours of immersion in water.

Conclusion

Medium-density fiberboard (MDF) panels
were produced in the present research project with two different resins of
urea-formaldehyde (UF) at 10% content, and isocyanate (IC) at 5% content. In
order to find new sources of fibers to satisfy the increasing needs for raw
material for composite industry, 10% palm leaf residues was added to the
composite mat to be compared with those panels with pure wood fibers. Moreover
nano-wollastonite was added to the mat at two contents of 5% and 10% to improve
some of the draw-backs of composite panels. Results showed that IC-resin has a
significantly lower water absorption and thickness swelling while mechanical
properties varied case by case. Addition of NW had a decreasing effect on the
mechanical properties of panels produced with UF resin, but it had an
increasing effect on panels with IC resin. Addition of palm residues had a
significant increasing effect on IB values in panels with both UF and IC
resins. It was concluded that palm leaf residues can be considered a potential
source of raw material for MDF panels produced with both UF and IC resins.
Moreover, NW is also recommended to be added in composite panels with 10% of
palm residues to compensate for part of the property losses.

Acknowledgement

The authors acknowledge constant
scientific support of Prof. Olaf Schmidt from the University of Hamburg, as
well as the support of Alexander von Humboldt Stiftung.

Koch J. W., 2006. Physical and
mechanical properties of chicken feather materials. A thesis presented in
partial fulfillment of the requirements for the degree of Master of Science in
the School of Civil Environmental Engineering; Georgia Institute of Technology,
May, 2006.